


Knocking Up Sister

by LoveRobin



Category: Kim Possible (Cartoon)
Genre: Big Sisters, Breeding, Brother-Sister Relationships, Brother/Sister Incest, F/M, First Time, Horny Teenagers, Impregnation, Incest, Porn With Plot, Porn with Feelings, Pregnancy, Sex, Shotacon, Sibling Incest, Sibling Love, Teen Pregnancy, Underage Sex, Vaginal Sex
Language: English
Status: In-Progress
Published: 2021-01-14
Updated: 2021-01-14
Packaged: 2021-03-12 12:07:40
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,300
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29010276
Author URL: https://archiveofourown.org/users/LoveRobin/pseuds/LoveRobin
Summary: Kim promises her brother Tim she will go to his elementary school prom as his date.Tim Possible gets a new girlfriend.
Relationships: Jim Possible/Original Character(s), Kim Possible/Tim Possible
Comments: 1
Kudos: 20





	Knocking Up Sister

**Author's Note:**

> Kim Possible and Co. are ©Disney, McCorkle & Schooley  
> All treatments are ©LoveRobin

_**Knocking Up Sister  
**_Chapter One — _Prom Night_

Most of the night, Tim—looking sharp in his deep red suit—escorted Kim on one of his arms, switching off as occasion warranted, as he proudly made a display of showing off his date.

"Dude. She's your _sister!_ " pointed out one of a clutch of his classmates with a sneer to his tone barely heard over the music.

"So what?" her brother replied.

Dressed in a long, slinky black gown with matching upper-arm opera gloves, a large corsage of orange blossoms on her left wrist. Her lithe left leg was visible through a long slit starting at the hip and sporting sparkling red three-inch pumps. His redheaded sister stood quietly at his side as she had been all night while he proudly showed her off like a prized show horse.

_Should've worn a ponytail._

__She did not care for the mental image, but as it was only in her head, she said nothing, watching as her brother put the others down.

"Don't try to act as if your date isn't your cousin," Tim countered before moving on to the rest of the clutch. "Or that yours isn't your mother. Yours is one of your younger sister's friends, _three grades younger_ ; okay, so she's tall for her age, dude, but still! If I recall correctly your date is your babysitter.

"Word has it your father got one of his employees' to put up a daughter in exchange for securing a promotion for her father, and yours is one of the mall food court taste samplers you begged to be your date tonight. How much besides her dress and shoes are you paying her?"

With a grin, Tim led away his smirking sister, jade eyes sparkling with delight at the snarky set of verbal takedowns worthy of Shego.

"Brutal."

Assessing his sister, Tim excused himself, momentarily leaving her standing near one of the sets of the _Madison A. Middleton Elementary School's_ gymnasium doorways. Even without her brother at her side, Kim still felt she was on display.

"Here. You looked thirsty," Tim returned with a couple cups of punch. Offering one, "I cut it, half water, just as you like it."

Duly impressed, the teenager took her less sugary drink. "Thank you."

He had been attending to her needs the entire night, having done the same with bringing her a plate of food, removing it when done, seeing she got a slightly larger slice of cake, and refills for her drink.

She was so appreciative, she did not move or fuss as her brother—in full view of his classmates—dropped his arm from her waist to possessively cup her left buttcheek. Kim was not only highly aware of her brother's thumb stroking her, but of her braless nipples popping to attention.

After fifteen minutes, "Almost done showing me off?"

"Almost." Tim firmly grabbed his sister's left ass-cheek. Another slow burn of excitement was sparked within her as she stood still, not calling undue attention to her brother squeezing, rubbing, and thoroughly enjoying feeling her up before stopping to stand there, the fingers of his left hand rhythmically brushing and patting her ass in full view of any who passed.

"And now?"

"Yeah, okay. I'm done for now."

"Why do you feel the need to degrade me like this."

"No degrading! Honestly, Sis, how could I not let people see that the loveliest, sexiest woman here is my sister and on my arm _as my date_.

"I can't wait for later. You'll love the hotel suite. I reserved the long weekend for us at the _Tipton_."

"Mighty sure of yourself, Bucko."

"I held up my end of the bargain. As promised, all pranking stopped and best behavior for two months for you to be my date."

"MM hmm," Kim changed the subject, "Where's Jim gotten hisself?"

Tim scanned the gymnasium. "There," he pointed, "Miss Adder's kissing booth. He's kissing her now."

"Whaa? I don't see hi—Oh. Never mind."

The lovely elementary math teacher was standing behind her booth as if waiting for customers—somehow she convinced the PTA that her selling out kisses to students once a year was better than them lusting after her all the time. Keep expectations under control, she claimed.

Her brother Jim was not visible unless one looked very carefully, the darkened lights helping to hide him. Unless one took note of her flushed expression and subtle movements under her skirt.

With an expert eye, Kim estimated he was on his knees, she half-sitting on his shoulders, his face in her crotch.

"Daring bastard."

"You've no idea. They've been fucking for months. They've had one scare already, but to her relief and Jim's disappointment it was a false positive."

"She's pretty. Sexy even."

"But she doesn't hold a candle to you. I shall maintain to my dying days the prettiest, sexiest woman here tonight is you. My date."

"I certainly applaud your tastes. I'm not sure how much the flattery will get you, but it's not hurting anything."

"We'll catch up to them later at the suite."

A hand once again on her ass, only with his two longest fingertips pressing past her taint, her brother, her date, guided her onto the dance floor.

Heedful of the escalated prompt, Kim nickered under her breath.

After about an hour, during which a savvy professional photographer quickly negotiated permissions to setup an impromptu secondary alcove to take premium 'Pictures with World Hero Kim Possible'—proceeds given to charity—ate into a lot of the siblings' 'date night'.

Kim had to promise a nearly sullen Tim she would make it up to him, "I'll make it worth it, wait and see", as well as no one— _none of the boys_ —got to stand as close to her as he had. There was always light visible between her and the others.

Eventually, the dance over, house lights were raised and volunteers unleashed to begin cleaning as music gave way to clashing conversations and laughter.

Finally, his sister again on his arm, Tim escorted Kim into their non-stretch limo to depart amidst the last flashes of paparazzi.

Tim eased himself closer to his sister.

"What?" Kim turned to look at her brother through half-lidded eyes.

Arm about her shoulders, Tim pulled her closer, leaning in to kiss his sister full on the lips.

_Oh boy. Time to pay the piper._

__Kim tried to relax into the awkward attempt at intimacy even as she felt her brother's hand slipping into the slit of her dress, crossing her thigh in search of her treasures. A slight push on her leg and she parted knees, giving him tacit permission to proceed further.

The limo driver pulled over at a prearranged corner, picking up Jim and Miss Adder far from prying eyes.

Seeing his siblings already busy and engaged and the progress his twin was making, Jim grinned.

 _"Tipton Middleton_ ", the boy ordered and as the car once again pulled out into traffic. Subservient teacher and her dominating student resumed their own fun. "Where were we, Addison?" She giggled and pulled intimately close to him.

As Tim's hands navigated between her legs, Kim had a fast conversation with herself. It only took about two heartbeats.

_How did I get in this position?_

___You promised him the 'complete girlfriend prom night date experience'._

___When did_ elementary school _proms start including overnight hotel stays?_

___Since large donations from rich entitled families._

___He's my_ brother.

_You also promised you'd make the photoshoot delay 'worth it'._

___But… doing my_ brother?

_Who says 'I can do anything?'_  
_  
No fair, using my motto against me!_

___Doing your brother is doing 'anything'._

__Sighing into the kiss with her brother, Kim leaned back to enjoy his fumbling yet earnest ministrations. Once her brother's hand discovered she was commando that night, she spread her legs as wide as the gown allowed, further opening herself to her brother.

Another moment of fumbling and Kim mewled as her brother's fingers found and slipped inside her snatch. She knew he could feel her wetness, she just hoped her dress was not showing an obvious damp spot.

"T–Ti _m–mmm_ …"

Enthusiastically, her brother's fingers had explored her forbidden area. Feeling the curls of her landing strip, he went lower, finding the shape of her lips, caressing up and down, brushing her clitoris, causing her hips to jump, making her increasingly wet and reactive. He probed, prodded, pulled, and frigged two fingers in and out of his sister.

_uhuunn_

__Tim placed his sopping wet and fragrant fingers to his sister's mouth. She tried to shake her head 'no', before sucking her taste from them like a baby with a pacifier. He dug back into her source, again she licked them clean.

Tim had his sister a wet panting mess by the time the limo pulled up to the hotel. Both couples pulled themselves together enough to pass for decency, Addison helped to rough and frazzle Kim's hair enough to prevent easy recognition.

Not that it was really needed. The _Tiptons_ were known for their discretions and customer privacies.

Check-in was fast, painless, and with keycards coded found the quartet in an elevator for the second penthouse suite. Once in the rooms, they took a quick tour together, learning the layout of four bedrooms each with bathrooms off a central sitting room with kitchenette and access to three balconies, two with a hot tub each and the third a full swimming pool.

After that, the two couples retired to their selected rooms, Addison Adder already stripping according to 'Sir's' directions.

"What's her deal?"

"Addie? Bro approached her in a supermarket after school. Straight up asked her for a date. Said he could make her cum more in an hour than she ever had before. She took it as a dare, thinking to shame him."

"Wow. I trust you'll not be trying that 'Sir' routine with me. Cause if you do, our deal is off."

"Oh, no! Never. I'd never treat a woman like that, even if willing. I think it's disrespectful of women. And _definitely_ not with you. I know you're nothing like that and how lucky I am to have you as my date tonight."

"Good." Shoes in her hand, Kim twirled so her slitted dress swirled, showing off her wet and dripping landing-stripped snatch.

She tossed herself onto the luxurious looking bed. She bounced twice, the first her body fully cleared the bedding. _"Niiccee…_ "

Kim swung her legs off the side to sit up. She pinned her brother with an even gaze.

"To be clear, when we made the deal and I promised to be your date tonight, I wasn't thinking it'd include an overnight hotel stay and what I suspect is me giving you _full_ access to me and my naughty bits."

"You promised—"

"—The _'complete girlfriend prom night date experience…',_ Yeah, I remember. I admit to not thinking it through—even ignoring Monique's best efforts to make me properly discuss it."

Grinning, "She's a sharp one, Sis. You really should listen to her advice more often." 

"Yeah. I really should… So. From what happened in the car… you want to fuck me."

"Very much so."

"Your sister."

"Please and yes."

"You want to fuck _me_ , your _sister_."

"I love you, Sis."

"You want _me_ , your _sister_ , to fuck you…"

"You promised…"

"You want _us_ to _fuck_."

"I want _us_ to _make love."_

"Brothers and sisters shouId'n—"

"'You can do _anything_ you put your mind to–" Kim paused, blinking as her brother played the Possible Family Credo card against her.

"–and I've put my mind to you."

Most were unaware Kim had shortened and adopted this, the second line, as her own personal motto. It made sense her brothers were just as inspired by it.

Then he invoked the third line: "'Keep all Possible promises'."

Capital-P Possible; promises to each other within the family before all others. Kim closed her eyes, already anticipating the response. "Tim, what you want us to do is not normal."

"'Anything is possible for a Possible'."

There it was. The first line of the family credo. The soul of their way of life. The way their father used it, it was virtually an immutable law of nature. Like Gravity.

"Sis, I know it's not normal for normal families, but we're _Possibles_. We don't do 'normal'."

"And that's it? You want to fuck me, so we fuck?"

"Nooo. I love you, Kim. I want us to _make love_.

"Well then, let's get started while I'm still in 'date mode' and you're saying just the right things. I've been _savagely_ turned on since the dance and what we were doing in the car." Kim stood, turning her back to her brother and lifting her hair from her neck.

"Unzip me. Take me."

Tim leapt to his feet and worked at her zipper. A first for Kim, she had never allowed her brothers to zip or unzip her outfits. Fortunately, their mother had given the boys ample opportunities, so her brother expertly had her back completely open in short order.

Stepping closer, he slipped his arms into the dress, reaching around to cop a feel of her boobs. Hefting and kneading, he also pinched and twisted at her nipples.

She knew how good her boobs could feel, she just never expected how much _better_ for someone else to handle her girls.

Having accepted the moment for what it was— _what it was going to be_ —his sister very quickly started to moan, pant, and rotate her hips.

_Oooh… ooooO_

__Her brother sent his right hand in search of her still very damp pussy. He fed her wetness to be licked away.

_hYa hYa hYa Mmmm niiice_

__Tim stood back and helped her gown fall into a circle at her feet. When she made to remove corsage and gloves, her brother asked her to leave them while guiding her back to sitting on the bed. Taking her shoes, dropping them atop her gown at their feet, he leaned over to kiss her mouth again.

His sister responded meekly, following her brother's lead. He licked across her lips with his tongue until she opened her lips so he could shove it into her mouth to find and duel with her own. Once she started moaning into his mouth, he broke off the kiss, leaving his sister panting heavily as he pushed her back onto the bed.

In a flash, her brother stripped down as naked as she was, his cock pointing straight at her, turgid and at the ready. She watched as he climbed onto the bed, placing hands on her knees to spread her legs. He knee-walked until he rested his rather impressive eight-inch erection, bobbing in front of him, along the length of her slit.

He took several minutes to plunge two fingers in and out of his sister. As hot as she was getting, with a devilish lilt to her lips, Kim made a game of tossing her hips about, her smoldering eyes pits of jade fire watching her brother expertly thrusting his digits into her seeping honeypot no matter what. There was no 'escape' for her.

Finally, he put an end to her lusty playfulness by flicking his thumb across her clit, sending a mewling convulsion of pleasure up her like a wave. He rubbed his junk across her wetness, lubing up for what was coming next.

With a grip on his cock, he rubbed along her slit, looking for the opening to her treasure, "I love you, Kim. I love you more than—"

His tip hooked on the opening he sought.

"—my next breath… _ohhh, fuucckk yyeess_ —"

Kim was unsure what to expect. A few months back for her birthday, Tara had gifted her what seemed to be a rather impressively complete kit of sex toys and paraphernalia, including several types of dildos and vibrators. Aside from briefly turning on a pink plastic torpedo which loudly buzzed, causing her to drop it before grabbing it to shut it off amid squeals and laughter from the squad.

As a dare she gingerly sucked on a large realistically molded silicon phallus and tried inserting it. She only got two inches in before abruptly returning it to its case as if she expected it to bite her hand.

That was all she had to compare with as her brother lowered his body atop her own, his eight-inch member pushing into the juicy flesh of her vaginal canal, filling her in a way she had no idea she had been empty.

 _hwoo_ _hwoo_ _hwoo_ _hwoo_ _hwoo_ _hwoo_ _hwoo_ _hwoo_

 __Kim micro-panted as her brother's dick— _My first real one!_ —slid home into her.

Once he came to a rest on her, head resting on her slight breasts, she hugged him with still-gloved arms, lightly holding him in place.

She rolled and undulated her hips as she cataloged the sensations of her brother inside her.

"Kim, Kim… _oh fuck_ , Sis, _oh fuck_. I never expected to feel _this, like this!_ I'm so glad yours is my first pussy… _fuck Sis!_ "

His sister patted his shoulder as she got her breath under control.

"M–my s–ister…, This is my sis–ster…, I'm inside my—you're _m–my sister!_ So–s–so _warm_. Inviting. Better than I ever imagined. Pressing from all sides… your pussy is grabbing at my cock, massaging it like you want to pull me in deeper."

 _mmMmm_  
"It _does_ feel nice. I'm glad you like it. From what Wade and Monique tell me, you're where a lot of guys want to— _hwwoooo hoo_ "

Her brother had pulled his dick almost all the way out, lifting to support his weight with his arms.

"W–wha–wha'er you doing? We finished already?"

"Far from it, Sis."

Her brother paused a moment as both his dick and his sister's pussy got used to each other.

"We're going to fuck."

 _hyhh_ _hyhh_ _hyhh_  
"I thought we _were_ fucking."

Her brother smiled down at her. _"This_ is fucking…."

He pushed his cock back into his sister, deeper than a minute prior, bottoming out as he bumped her cervix.

_hwooo_

__Pulled out. Pushed in. Pulled out. Pushed in.

 _hooh_ _hooh_ _hooomigawd!  
__"THIS_ is fucking?"

"Some shit, huh?"

"Holy fucking shit!"

"Hang on, Sis," her brother increased the pace.

Pull out, push in, pull out, push in.  
Pull out, push in, pull out, push in.  
Pull out, push in, pull out, push in.

"Faster. Harder," Kim's hips rose to meet her brother's thrusts.

"As you wish," Tim picked up the rhythm, throwing his cock into his sister's pussy with gusto.

Her brother slid in and out of her as if to the beat of a fast tempo track used for timing at cheer practice.

_awhooooo_

__Lifting at the knees, Kim rested her legs along her brother's heaving sides, rubbing them. She looked like a jockey riding _under_ her mount.

"Sis, Sis! I love your pussy! I love fucking your pussy!"

"Ohmigawd, Tim. I love your dick—"  
"Cock."  
" _—cock_ … I–I love your cock fucking my pussy!"

"It's like we're made for each other, Sis."

"…you're… _made_ for me?"

"For you and you only."

_mmMMmm hAAh hAAh_

__"…f–for m–me… _hAAh hAAh_ …only…"

Tim felt his cock tingling. "Sis… oh, Sis… I can feel it."

"I'm feeling it alright."

"Cum–cumming… I can feel it, I gonna cum!" The sensation grew.

_hAAh hAAh YeeEESSss_

__"Do it!"

Getting stronger, "I'm gonna cum inside you…"

"Do it, Bro. Cum inside your sister's pussy."

"K–Kim–m, Kim, I love yo—CUMMING! CUMMING! Oh fuck I'm cumming in my sister's pussy!"

The tingling reaching a cresendo, Tim erupted. Spurt after spurt he squirted his sister's insides, her sheath rippling along his length as if in gratitude, trying to milk him dry.

Brother and sister lay atop sweat-drenched sheets, panting for breath, she rubbed his right side with her leg.

"Well, Sis?"

"Mmmmm, that was nice."

Tim stretched to kiss his sister. She enthusiastically returned it.

"Are you getting hard again?"

"Yep." He kissed at the corner of her mouth. As they talked, his sister returned each and every one.

"You haven't even pulled out yet."

"Ready for more, Sis?"

"Hmmm… yes."

Smiling widely, her brother somehow managed without pulling out of his sister to maneuver her legs straight up between them, having her hug her arms at the knees, calves just over his shoulders and to either side of his head, allowing them to look each other in the face.

Tim used his sister's legs as a platform against which to lean his form, revived cock still deep in her folds.

"Ready, Sis?"

Once again Kim wiggled her hips, cataloging the feel of her brother pressing against her cervix.

"Yeah. Fuck your sister—"

"I'ma make lov—"

 _"Fffuck_ your sister!"

"Well, then. Here we go!"

Digging his feet against mattress and bedding, he drove hips against her as if to move his sister bodily. Her ass lifted a couple inches as his cock pushed inside his sister an extra couple inches deeper against her womb.

_hUuuhnn_

__"Like that?"  
_  
"YEeessss."_

Pulling halfway out, her brother began a steady, purposeful, cyclic motion of his hips; pulling back with an upward stroke before dropping into a smooth in-stroke, hardly pausing before repeating.

_hyWoooooo_

__After several firm and steady back-up-down-in strokes, an almost indecent squishy wet sound from his sister's filled and juicy pussy filled the room along with her moans.

 _skwssh_ _skwsshee_ _skwssh_ _hWoooooo_ _skwsshee_ _skwssh_

 __ _"YyeeesSS_ T _iiimmm!_ _fFffuuck this pussy! FuUCK your sisssster!"_

 __ _"hhh_ Enjoy, Sis! Since I came once already, I'm gonna last a while with this one. Gonna build up a nice heavy load of my seed for ya! Gonna fill y'up…"

 _skwssh_ _skwsshee_ _skwssh_ _hWaaaaatt?_ _skwsshee_ _skwssh_

 __"S–seed?"

"Yeah. Sperm, cum, seed, cream, spunk, jizz—"

Kim flexed her kegels, tightening down on her brother's pistoning prick. Not with her full strength, not wanting to hurt him, only to slow him down some. And while it _did,_ the additional resistance also _enhanced_ the sensations both felt as he continued—with increased effort—to stroke in and out.

She spoke as she moaned in pleasure, "S–seed makes it sound l–l _iiiii_ ke you're trying to make me p–pregnant."

"Of course, I am," his rhythm was still firm, her resistance having them both feeling good.

"W–w–wha? Y'can't impregnate me!"

Pull-up-down-push.  
Pull-up-down-push.

"I assure you, Sis, I can. And I will."

"T _iiii_ mmm…"

"We're fucking now and we're not going to stop— _fuck, you're so tight_ —We'll be fucking _a lot_ from now on. Every minute we can, as often we can…"

"Y–yer talkin'bout _breeding_ me."

"Sure am. Also often."

"Thass n–not possi–," Kim stopped, realizing exactly what she inadvertantly queued up for her brother.

"Anything is possib—"

"Yeah, yeah, you fucker."

Tim stopped moving. He looked his sister in the eyes with concern.

"Look, Kim, I love you with all my heart and soul. Mom says children are the ultimate expression of loving and making love. I'll _take_ responsibility. Our children will beautiful, smart, and as special as their mother.

"But true love doesn't _force_ things. I'll not _rape_ you.

"You want me to stop… stop loving you, stop proving my love for you… I'll stop. It'll be hard, but I _will_ stop.

"Just tell me to stop."

Kim searched her brother's expression and only saw sincerity and love. Through all their history of annoying pranks, she had never doubted the familial love core of the Possible Family.

After a long pregnant pause—

_Ha! Puns. Et tu, sense of humor?_

__—Kim's body answered for her; her hips were throwing themselves at her brother's cock, her vaginal muscles rippling along the shaft as if she could pull him back into action.

Her resistance, both kegel and philosophical, faded away.

Unhugging her legs, the redhead's calves draped over her brother's shoulders and she pushed on his back with her heels.

"Well, you fucker, what're you waiting for? No slacking off on the job. Y'can't breed me like this!"

"You sure, Kim?"

Hands on his shoulders, Kim's body tried to climb his.

"Shut up and start fucking babies into your sister."

Smiling, her brother resumed pistoning in and out of his sister, plumbing her depths with each squishing in-stroke.

_mmmMMmmm fuck yeah mama needs this mama wants this_

__Pull-up-down-push.

 _"Fuck me fuck me fuck me,"_ she urged him, sounding like she was cheering.

Pull-up-down-push.  
Pull-up-down-push.

_fuck fuck fuck your sister_

__"Oh. Oh, yeah… al–most… there, Sis!"

His sister felt her climax building as well. She energetically threw herself into meeting his thrusts.

"Come on, Fucker, gimme your load of seed! Fill your fucking sister t'overflowing!"

"It's… I'm cumming… Oh, Sis, _it's gonna be a big one!"_

___fuck me breed me fuck me breed me_

___"Here I CUMmmm!"_

___fuck y'sister breed y'sister fuck y'sister breed y'sisTeeeeeR!_

__Brother and sister climaxed together. His sister's vaginal walls clamped and rippled, clamped and rippled as her brother's cock spewed forth, basting her insides with rope after rope of his baby-making seed.

Their bodies stiffened in mutual pleasure. He trying to shove himself into his sister as his sister tried to pull him inside herself.

Finally they went limp. Sweaty and huffing for air. Sister holding brother close.

"Are you… still cumming?"

"Fuck yea, Sis. Your pussy is milking me dry. Oh, fucking fuck yeah."

She bent to kiss her brother. "Well, as promised, one girlfriend."

"For just the weekend stay?"

Kim giggled, kissing her brother again, this time very deeply.

"Of course not. Breeding me the way you want to is going to take a while.

"A long while…"

"Sis, this is the start of a beautiful relationship."

"How soon c'you fuck me again?"

**Author's Note:**

> My first smut, but not the only one I'm working on.
> 
> Yes, I've altered my style of writing to be very repetitive with "his sister" and "her brother". I intentionally elected to strongly emphasize their illicit relationship more so over any other "normal" stories and plotlines. Grammar Nazis may stand down.
> 
> Addison "Addie" Adder is the name I've given to a canon background character. She was the Pre-K teacher in ASiT Past, and later teaching or substituting an MHS Math class in Bonding. Probably teaching as she administered a test, something Subs rarely do unless assigned for more than a week. I figger iffen she was 18-20 in Pre-K when Kim was 4, in MHS, and for this fic, she's 28-30. Perfect age for a MILFy submissive for a Jim.


End file.
